Electrical heating device

ABSTRACT

The present invention relates to a PTC based heating device for a motor vehicle, preferably with electrical propulsion, in which one or a plurality of supplementary circuits are connected to a heating circuit for being switched in optionally. In this way it is possible to use a large number of possibilities for temperature control for energy management which is important in electric vehicles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrical heating device for a motor vehicle. In particular the present invention relates to an electrical heating device with a fluid medium as heat transfer medium which is suitable for electric and hybrid vehicles.

2. Description of the Related Art

With regard to heating there is a problem with vehicles with electric or hybrid drive in that the drive unit of the motor vehicle does not dissipate sufficient waste heat for the heating or air conditioning. An appropriate vehicle heating system must therefore be suitable for providing both the interior of the motor vehicle with the required heat for heating purposes as well as providing the heat required for or at least supporting the running processes in the individual system parts of the motor vehicle, such as for example for preheating the vehicle rechargeable battery. An obvious method would be to provide the required heat by electrical means. In the state of the art it is known that so-called resistance heating elements or PTC (Positive Temperature Coefficient) heating elements can be used for this purpose. They are self-regulating, because they exhibit a higher resistance with increasing heating, thus passing a lower amount of current. The self-regulating properties of the PTC heating elements thus prevent overheating.

Accordingly PTC heating elements are often used in heating devices which are particularly used for heating the vehicle passenger compartment in vehicles, the drive of which does not produce sufficient process heat for the air conditioning or heating system of the vehicle passenger compartment. With hybrid vehicles a PTC heating device can also be used as an auxiliary heater in the phases in which the internal combustion engine is not running (e.g. at traffic lights or in a traffic jam).

When electrical heating elements are employed in a vehicle with electric propulsion there is a further problem in that the vehicle battery (accumulator) acts as the energy source both for the vehicle drive to provide traction as well as for the heating. Thus, the vehicle drive and the heating are competing for the limited amount of energy which is available between two battery charging processes. Expressed in another way, the vehicle operating range between two battery charges reduces with the electrical energy used for electric heating.

Therefore, in an electric or hybrid vehicle a well thought-out concept of energy management plays a central role in which the drive, the heating and other components consuming electrical power, such as the lighting, are taken into account. An important aspect of energy management is the use of additional energy resources in the vehicle which are not based on the electrical energy of the battery. Resources of this nature are available in the vehicle in particular in the form of waste heat from certain vehicle components. Components, which make additional energy available in the form of waste heat include an additional internal combustion engine on hybrid vehicles (“range extender”), brakes and the vehicle battery itself or a built-in battery charger (on-board charger). It is therefore desirable to exploit the waste heat arising in the vehicle within the scope of the energy management system as far as possible for the heating in order to extend the operating range. With PTC heating the air in the vehicle passenger compartment is heated with the aid of the PTC resistance heating elements either directly (air heater) or indirectly via a circuit in which a hot liquid flows through radiators. Preferably, water is used as the liquid (hot-water heating). For the energy management system hot-water heating has the advantage that the waste heat taken up by water in one region of the vehicle (cooling circuit) can be used by cooling and heating circuit for heating at another location.

It should be noted however that waste heat from vehicle components is generally only available temporarily. Thus, an additional internal combustion engine on a hybrid vehicle only supplies waste heat when it runs. Braking supplies a significant amount of waste heat, in particular during longer downhill sections. With regard to the vehicle battery the situation is such that it must first be brought to a required operating temperature by preheating and is then cooled during running operation. The combination of heating and cooling circuits is thus in particular a disadvantage if with the lack of waste heat, the presence of a large quantity of cold coolant in circulation prevents efficient heating and thus negatively affects the electrical energy consumption and operating range.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a heating device for a motor vehicle, in particular with electrical propulsion, which facilitates efficient energy management.

According to the present invention, an electrical heating device for a motor vehicle, in particular with electrical propulsion, is provided. A liquid medium is used with the heating device as the heat transfer medium. The heating device comprises a heating circuit with a liquid medium as the heat transfer medium. Furthermore, the heating device comprises an electrical PTC heating element arranged in the heating circuit for heating the liquid medium. Moreover, the heating device comprises a supplementary circuit with a separate heat source which can be optionally switched into the heating circuit.

It is the particular approach of the present invention to arrange a heating device with an electrical PTC heating element such that additional heat sources made available can be used for efficient energy management. This is achieved in that a supplementary circuit, which makes heat available separately from the electrical heating element, can be switched in optionally. By controlling the switching in and out of the supplementary circuit, depending on the heating requirement and operating conditions, the supplementary circuit can be included in the energy management.

The heating device may also comprise a control device for controlling the PTC heating element. This enables the PTC heating element to be controlled such that it bears a proportion of the total heating power which cannot be supplied by means of the supplementary circuit. It is thus achieved that the heating is supplied as far as possible from non-electrical energy sources, in particular from waste heat. The electrical heating is used when the heat from other sources is not sufficient.

The alternative switching in of the supplementary circuit may be controlled such that the heat requirement of the heating device is first of all covered by switching in the supplementary circuit. The supplementary circuit is controlled such that it is switched in when the heating device has a heat requirement which can be covered by the supplementary circuit. According to a preferred embodiment, the optional switching in of the supplementary circuit occurs with the aid of valves.

Further, the heating device may additionally comprise a pump with which circulation of the liquid medium is produced in the heating circuit. The circulation of the fluid medium in the heating circuit is used for the transport of the heat provided by the electrical heating element and, where applicable, further heat sources. This can occur in that the pump is only switched on and off as required, or also by additional control of the pump power. Generally the following applies: the higher the heating power, the larger the flow velocity in the circuit must be. With large heating power requirements a larger amount of heat can be transported with a higher rotational speed.

The separate heat source of the supplementary circuit may be the waste heat from a component of the motor vehicle. In this way existing heat is efficiently exploited which would otherwise have to be uselessly dissipated. Therefore the cooling of one component or a region of the vehicle can be linked with the heating of another component or another vehicle region. This is employed to save the electrical vehicle energy and thus to increase the operating range of the vehicle. Heat may be made available by the internal combustion engine of a hybrid vehicle. An internal combustion engine of this nature can in particular be a so-called “range extender” which is particularly switched in for longer trips in order to increase the range. A range extender of this nature can in particular be used in the serial hybrid design, whereby the internal combustion engine (range extender) produces electrical energy, which can be used via a generator directly for the vehicle drive as well as indirectly for battery charging. Combinations are possible through alternative switching. Alternatively, a range extender can also be switched in via a clutch as a direct drive for vehicle propulsion (parallel hybrid design).

Further conceivable heat sources for feeding the supplementary circuit are the waste heat from the brakes, in particular on longer downhill sections, and the waste heat from the vehicle battery. With vehicle batteries used for electric and hybrid vehicles in the automotive high voltage range of a few hundred volts (e.g. 300 V, 380 V or 500 V) the continual cooling of the battery during operation plays a significant role. On the other hand, when putting the vehicle into operation it is initially necessary to preheat the cold battery to a certain operating temperature. Within the scope of the present invention it is, for example, possible to use the valve control of a supplementary circuit initially for preheating the battery, and when it has reached operating temperature, the battery waste heat is used for heating the vehicle passenger compartment.

In the vehicle configuration the heating device according to the invention may comprise further supplementary circuits which can be switched in optionally.

The heating device furthermore may comprise a temperature probe (temperature sensor) arranged in the heating circuit, whereby the heating device is controlled such that a temperature measured by the temperature probe is adjusted to a specified temperature target value. Furthermore, the control device may comprise a comparator, which compares the temperature measured by the temperature probe to a temperature target value. Within the framework of the energy management system according to the invention, if the temperature is to be increased, initially a status check may occur of whether a supplementary circuit is able to supply sufficient hot water. It is only when this is not sufficient that the power of the PTC is increased. For the case in which the temperature is to be reduced, initially the PTC heating power may be reduced and then cold water from a supplementary circuit is supplied if necessary for faster cooling.

Further, a supplementary circuit may be switched off when it cannot make heat available for heating. This is particularly important when the electrical heating must be called upon for heating. In the cold vehicle state, for example before the start of a trip, heating up takes place more efficiently the less cold water there is initially in circulation which has to be heated.

In a heating device according to the present invention the PTC heating element may be controlled in dependence of a predetermined heating power. Also, the predetermined heating power is here equal to the difference of a required total heating power and a heating power which can be covered by supplementary circuits.

According to an embodiment, one or a plurality of PTC heating elements, a control device, valves for switching one or a plurality of supplementary circuits in and out and a pump for generating a circulation of the fluid medium may be arranged in a common housing. A form of construction of this nature facilitates a particularly compact heating device, in which in particular the elements required for the control are integrated into the heating device such that their available processor capacity is optimally exploited and separate control units are superfluous. This facilitates a higher integration and reduces the general complexity of control.

Preferably, water is used as the liquid medium (heat transfer medium).

Further advantageous embodiments of the present invention are the subject matter of dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following based on the accompanying figures in which:

FIG. 1 illustrates a heating circuit of a heating device with a supplementary circuit according to the present invention, whereby

FIG. 1A illustrates a state in which the supplementary circuit is separated from the heating circuit; and

FIG. 1B illustrates a state with an open (included in the heating circuit) supplementary circuit;

FIG. 2 illustrates a representation of a heating circuit according to the present invention with a further supplementary circuit;

FIG. 3 shows a schematic representation of a control concept for a heating device according to the present invention;

FIG. 4 illustrates a flow chart for an embodiment of the control of a heating device according to the present invention; and

FIG. 5 illustrates a further flow chart for an example of a control of a heating device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a motor vehicle heater which may be formed as a hot-water heater and comprises one or a plurality of PTC heating elements. If a plurality of heater elements are present, they may be combined to form one or a plurality of heating stages, whereby the heater elements of a heating stage are controlled together. A control is for example possible with the aid of pulse width modulation (PWM). Furthermore, a combination of a plurality of binary heating stages is possible which can be adjusted quasi-continuously or in fine steps using PWM. Thus, a finely stepped adjustability with minimisation of the undesired side effects present in the automotive high voltage range such as EMC (Electro-Magnetic Compatibility) interference can be achieved.

A heating device according to the present invention facilitates heating of the vehicle passenger compartment and, where applicable, further vehicle components, whereby apart from the electrical energy (via the PTC heating elements) further (preferably non-electrical) heat sources can also be used. The latter furthermore may involve waste heat from vehicle components which have to be cooled and the waste heat of which would otherwise be lost and unused. To achieve this, the heating device comprises according to the present invention, along with a main circuit (heating circuit), one or a plurality of supplementary circuits, which are switched in optionally. With the aid of a suitable control device the optional switching in and out occurs such that the heating device supports efficient energy management of the vehicle. In this respect, it is achieved that the electrical heating power is only drawn on when there is not sufficient heat available from other sources. This contributes to an efficient exploitation of the electrical energy and thus to as large a vehicle range (operating range) as possible.

With the objective of reducing the general outlay for control, the processor capacity, which is necessary in any case for the control of the electrical heating device, may be assigned additional tasks which provide control of the supplementary circuits.

An example of a construction of a heating device according to the invention with a heating circuit (main circuit) 2 and a supplementary circuit 3 is shown schematically in FIG. 1. FIG. 1A illustrates the supplementary circuit in the state shut off from the main circuit and FIG. 1B shows the opened supplementary circuit, so that the fluid heat transfer medium (designated as water as an example in the following) flows through the supplementary circuit 3.

The heating device 1 comprises a main circuit 2 and a supplementary circuit 3 connected by means of 3/2-way valves 40 a and 40 b.

Arrows in the circuit symbolise the water flowing through the circuit. For reasons of safety in the motor vehicle the maximum water temperature is preferably 60° C.

In the main circuit there is also a PTC heating element 20, a radiator (heat exchanger) 30 and a pump 60. In a preferred embodiment illustrated here the valves 40 a and 40 b are arranged in a part of the heating circuit 2 located opposing the direction of flow relative to the PTC heating 20. However another arrangement is also possible. An additional heat source 50, arranged in the supplementary circuit, is in a preferred embodiment an additional internal combustion engine (range extender) of a hybrid vehicle.

The power supply of the PTC heating element 20 is provided by the vehicle battery 10, which is also responsible for the traction (propulsion) of the vehicle. In the heat exchanger (radiator) 30 heat is dissipated from the water to the surrounding air. The radiator 30 can be arranged such that the air, for example in the vehicle passenger compartment, is directly heated. Alternatively, air can be heated, which for example is blown into the vehicle passenger compartment by a fan. Furthermore, it is possible to realise a combined direct and indirect heating of the interior air with the aid of a plurality of radiators. The pump 6, preferably driven electrically, maintains the flow of water in the circuit. Depending on the control of the valves 40 a and 40 b, the circulation of the water in the circuit occurs either directly through the short section of the main circuit between valve 40 a and valve 40 b (FIG. 1A) or however by inclusion of the supplementary circuit 3 (FIG. 1B). As shown in FIG. 1B, here the water flows starting from the valve 40 a, which is open in the direction of the supplementary circuit 3, initially through the supplementary circuit to the range extender 50. Here the water can, preferably with the aid of a further heat exchanger (not illustrated in the figure), take up waste heat from the range extender. Then the water flows through the supplementary circuit 3 via the valve 40 b, which is actuated appropriately, back into the main circuit.

Instead of the range extender 50, in the supplementary circuit another heat source can also be arranged, separate from the electrical PTC heating. With the aid of an appropriate heat exchanging device the waste heat from the brakes, for example, can be removed by the water flowing through the supplementary circuit.

In order to include the heating device 1 in the overall system of energy management of the electric or hybrid vehicle, an appropriate control of at least the PTC heating element 20 and the valves 40 a and 40 b is required. The control concept and the corresponding control device (not illustrated in FIG. 1) are explained in further detail with reference to FIGS. 3 and 5. The pump 60 may also be controlled by the control device, whereby either only its on/off switching or also the pump power (dependent on the rotational speed) can be controlled. From a standpoint of mechanical construction it is advantageous if the PTC heater is integrated into one single housing together with the valves, pump and control device. This saves space and is particularly cost-effective.

Within the scope of the concept of the present invention it is furthermore possible, through further supplementary circuits which can be optionally switched in, to supply certain regions of the motor vehicle specifically with heat (optionally included in the heating circuit). The valves for connecting optional supplementary circuits of this nature are preferably arranged in the direction of flow as viewed from the electrical PTC heating device.

In FIG. 2, along with the supplementary circuit 3 which can be alternatively switched in via the control of the valves 40 a and 40 b, a second supplementary circuit 3 a, which can be alternatively switched in with the aid of the 3/2-way valves 40 c and 40 d, is illustrated. In the example illustrated in FIG. 2 the supplementary circuit 3 a runs in the vicinity of the vehicle battery 10. A supplementary circuit of this nature can fulfil two functions:

On the one hand, the supplementary circuit 3 a can be used to preheat the battery until it reaches its operating temperature. For this purpose the supplementary circuit 3 a is included in the heating circuit by appropriate control of the valves 40 c and 40 d and has water flowing through it which is heated by the PTC heating element 20. In the phase of preheating the battery the valves 40 a and 40 b are preferably controlled such that the supplementary circuit 3 of the range extender is switched off. Since it cannot provide any heat before the vehicle is put into operation, it is advantageous for the rapid and efficient heating of the water in the heating circuit to reduce the circulating water quantity by switching off the supplementary circuit 3.

Furthermore, it is also alternatively possible to provide the radiator 30 for heating the vehicle passenger compartment also in a section of the circuit which can be switched in alternatively by suitable valves (not illustrated in FIG. 2). With an embodiment of this nature and for especially fast heating of the battery, the radiator 30 can be initially removed from the heating circuit and later, preferably once the battery has reached its operating temperature, switched in again. The supplementary circuit 3 a can be switched off at the same time.

On the other hand, the circuit 3 a can be used as an additional heat source in a similar manner as the supplementary circuit 3 illustrated on the left in FIG. 2. Here, the battery waste heat arising during the running operation of the battery 10 can be used as a separate heat source. For this purpose, the supplementary circuit 3 a is not separated from the rest of the circuit once the battery reaches its operating temperature, or it is switched in again at a later time during running operation, depending on the demand.

FIG. 3 shows an overview of the control concept of the heating device according to an embodiment of the present invention. An important advantage of the present invention is that the control device for the control of the valves and pumps can be integrated into the control electronics which are in any case present in a high voltage (HV) PTC heater 20. In other words the processor capacity of the control electronics already present in the PTC heater is assigned additional tasks, such as the control of the pump 60 and the valves (symbolised in FIG. 3 by the 3/2-way valves 40 a and 40 b). For this purpose the HV PTC heater also has, in addition to the high voltage connection (symbolised by the HV plug 22), connections in the automotive low voltage range (LV—low voltage—symbolised by the LV plug 24). These are used on one hand for the transfer of control data to the components (pumps and valves) which are external to the HV PTC 20. On the other hand control data is transferred to the control device via these connections. In the illustrated picture the air conditioning operating panel 70 of the vehicle is shown as an example data source. In this way it is possible for example for demands for a desired temperature or also power demands to be entered by the user. Data communications preferably occur via a vehicle bus (symbolised in FIG. 3 by a LIN bus). The invention is however not restricted to certain interface formats. Other bus systems can also be used, such as for example CAN bus systems. Via the control connections in the low voltage range (preferably via the vehicle bus system) further data relevant for the control of the heating device can be communicated to the control device integrated into the HV PTC 20. This relates, for example, to the provision of data which is determined by temperature measurement devices (temperature sensors) in the heating circuit or at other positions in the vehicle. Alternatively or additionally, with regard to the availability of heating energy (e.g. from waste heat) in certain supplementary circuits which can be switched in alternatively, data can be communicated over the vehicle bus and the LV plug 24 to the control device.

Embodiments of a control of a heating device according to the present invention are described in the following as examples based on the flow charts in FIGS. 4 and 5.

FIG. 4 illustrates a processing example in which a measured temperature is to be adjusted to a specified temperature target value. In this respect the measured temperature may be a temperature acquired by a temperature probe at a certain position of the heating circuit. Here, the temperature probe is preferably arranged, viewed in the flow direction, behind the PTC heating device. However, other arrangements are also possible.

Alternatively, a temperature may be involved which is measured in a region of the motor vehicle which is heated by the heating device. This may be, for example, the vehicle passenger compartment. It may however also be a temperature at another region of the motor vehicle, such as for example in the region of the vehicle battery (accumulator).

In a first step in the method (S10) the temperature target value T_(SET) is defined. This can be done by an operator (driver or vehicle occupant), for example with the aid of the air conditioning operating panel 70. In the second step in the method (S20) the control electronics integrated into the HV PTC heating device 20 checks whether the temperature currently measured by the temperature probe T_(ACTUAL) is lower than the temperature target value. If the measured temperature value is lower than the specified temperature target value (S20:Y), the method moves on to step S30.

In step S30 the control electronics carries out an assessment of whether a shut-off supplementary circuit can provide additional heat. Information of this nature may be made available to the control device via the vehicle bus. For example, the control device has information available about the available waste heat of a range extender (additional internal combustion engine on a hybrid vehicle) via the temperature of the range extender. Analogously, appropriate information for other sources supplying heat, such as for example brakes or the vehicle battery, can be made available based on their temperature.

In the case where a shut-off supplementary circuit can provide heat (S30:Y), in the following step (S40) the relevant supplementary circuit is switched in by appropriate control of the valves. The method then proceeds to step S60. In step S60 the control electronics assesses whether the supplementary circuit can provide sufficient heat to bring the temperature to the temperature target value T_(SET). If the assessment is positive (S60:Y), the method finishes according to FIG. 4. As shown by the arrow (S60:Y->S20), the processing cycle then starts from the beginning. In the alternative case (S60:N) the heating power of the PTC heating device is increased in the next step (S50).

If alternatively the assessment in step S30 reveals that currently no shut-off supplementary circuit is able to provide additional heat (S30:N), the method then moves directly to step S50. In step S50 the PTC heating power is increased by the control device.

If it is furthermore found in the assessment step S20 that the currently measured temperature is not lower than the temperature target value (S20:N), the method initially continues with the assessment step S70. In step S70 it is assessed whether the current temperature is higher than the temperature target value. If this is not the case (S70:N), i.e. the current temperature (within a specified tolerance as applicable) is equal to the temperature target value, then the method terminates according to the flow chart of FIG. 4 and the cycle starts again with the step S20.

In the alternative case (S70:Y), i.e. when the measured temperature is higher than the temperature target value, the method proceeds with step S75. In step S75 the control device assesses whether the PTC heating device is currently supplying heat. If the PTC heating device is currently supplying heat, at (S75:Y), then the method proceeds to step S90. In step S90 the control device assesses whether a currently shut-off supplementary circuit is present which can feed cold water into the heating circuit. If this is not the case (S90:N), the method terminates according to the flow chart in FIG. 4. In the alternative case (S90:Y) initially all supplementary circuits which can supply cold water (step S100) are switched in. Thus, fast cooling is achieved (aligning the temperature actual value with the temperature target value).

If it is assessed in the assessment step S75 that currently heat is being supplied by the PTC heating device (S75:N), the PTC heating power is initially reduced in the following step S80. Then the method returns to the decision step S20 and a further check is made of whether the reduction of the PTC heating power obtained is sufficient to equalise the measured temperature to the temperature target value.

Accordingly, the method is continuously repeated following the loops illustrated in FIG. 4, starting from the assessment step S20. Alternatively, the assessment can also be implemented according to the steps S20 and, where applicable S70, in specified regular intervals. This results in a corresponding waiting period between the end of one passage through the loop and the start of the next passage through the loop S20. A restart of the method according to FIG. 4, with step S10, occurs with each redefinition of the specified target temperature T_(SET).

Other modifications of the sequence illustrated in FIG. 4 as an example are possible within the scope of the present invention. For example, initially step S70 can be carried out to assess whether T_(ACTUAL)>T_(SET) and in the negative case the method continues with step 20.

A further alternative is that instead of a temperature (T_(SET)) a heating power to be obtained is specified. The control of the heating device including the PTC heating device 20 and the supplementary circuits 3, 3 a then occurs in that it is initially assessed which proportion of the heating power demand can be supplied from non-electrical heat sources via supplementary circuits. Within the scope of energy management the use of the PTC heating is restricted to the remaining part which cannot be supplied from other sources. Finally, it is also possible that a power specification has its origin in a temperature specification which is converted into a power specification by the control electronics.

In the following the method is described according to another embodiment for the control of the heating device according to the invention with reference to the flow chart in FIG. 5.

On putting the motor vehicle into operation after a longer period at standstill, the method according to FIG. 5 starts in step S200 with the PTC heating device 20 being switched on and the battery supplementary circuit 3 a being switched in to preheat the vehicle battery 10 through appropriate control of the valves 40 c and 40 d (refer to FIG. 2). Since with the vehicle at standstill no thermal energy is to be expected from supplementary heat sources, such as waste heat, the position of the valves for the connection of further supplementary circuits, such as for example, supplementary circuit 3 for the range extender 50, is preferably in the shut-off position. Thus, a faster circulation of water and therefore faster preheating of the battery are achieved.

Then in the assessment step S210 it is assessed whether the operating temperature of the vehicle battery 10 has been reached. As long as the operating temperature of the battery 10 has not been reached (S210:N), the status of the heating device remains unchanged (loop S210:N->S210).

When the battery operating temperature has been reached (S210:Y), the method continues to step S220. In step S220 the battery supplementary circuit 3 a is initially switched off in that the position of the valves 40 c and 40 d is changed appropriately. Then in step S230 the control device assesses whether a requirement on additional heat is needed in the circulation and whether the supplementary circuit 3 a of the battery can provide waste heat. An assessment of this nature can take place, for example, within the scope of energy management which is illustrated in FIG. 4 as an example. Alternatively, the assessment can also be implemented according to different criteria, for example, based on a certain heating power specification.

As long as no battery waste heat is available or required for heating (S230:N), the status of the heating device remains unchanged. Otherwise (S230:Y) the battery supplementary circuit 3 a is switched in again by the control device in step S240 through appropriate control of the valves 40 c and 40 d.

The present invention, as defined in the accompanying claims, is not restricted to the embodiments described in detail above. In particular individual features of certain embodiments can be combined, provided this does not lead to conflicts.

Summarising, the present invention relates to a PTC based heating device for a motor vehicle, preferably with electrical propulsion, in which one or a plurality of supplementary circuits are connected to a heating circuit for being switched in optionally. In this way it is possible to use a large number of possibilities for temperature control for energy management which is important in electric vehicles. 

1. A heating device for a motor vehicle, the heating device comprising: a heating circuit with a liquid medium as heat transfer medium, an electrical PTC heating element, arranged in the heating circuit, for heating the liquid medium, and a supplementary circuit with a separate heat source which can be optionally switched into the heating circuit.
 2. The heating device according to claim 1, further comprising a control device for controlling the PTC heating element, wherein the PTC heating element is controlled such that it contributes a proportion of the total heating power which cannot be provided via the supplementary circuit.
 3. The heating device according to claim 1, further comprising a control device for controlling the switching of the supplementary circuit, so that a heating requirement of the heating device is first of all covered by switching in the supplementary circuit.
 4. The heating device according to claim 1, wherein the switching of the supplementary circuit occurs with the aid of valves.
 5. The heating device according to claim 1, further comprising a pump for producing a circulation of the liquid medium in the heating circuit.
 6. The heating device according to claim 1, wherein the separate heat source of the supplementary circuit is supplied from waste heat of a vehicle component.
 7. The heating device according to claim 6, wherein the motor vehicle is a hybrid electric vehicle, and wherein the waste heat is made available from a range extender internal combustion engine of the hybrid vehicle.
 8. The heating device according to claim 1, furthermore comprising a temperature probe arranged in the heating circuit, wherein the heating device is controlled such that a temperature measured by the temperature probe is adjusted to a specified temperature target value.
 9. The heating device according to claim 1, wherein the supplementary circuit is switched off when the supplementary circuit cannot provide any heat for heating.
 10. The heating device according to claim 1, comprising at least one further supplementary circuit which can be switched in optionally.
 11. The heating device according to claim 1, wherein the supplementary circuit is switched in when the temperature in the heating circuit is to be reduced and the liquid medium in the supplementary circuit, which is switched in, is colder than in the heating circuit.
 12. The heating device according to claim 1, wherein the PTC heating element is controlled in dependence of a predetermined heating power.
 13. The heating device according to claim 2, wherein the PTC heating element is controlled in dependence of a predetermined heating power.
 14. The heating device according to claim 12, wherein the predetermined heating power is equal to the difference between the required total heating power and a heating power which can be covered by the supplementary circuit.
 15. The heating device according to claim 10, wherein the supplementary circuit, heats a specific region or a specific component of the motor vehicle.
 6. The heating device according to claim 1, wherein the PTC heating element, a control device, valves for switching a supplementary circuit in and out, and a pump for producing a circulation of the liquid medium are arranged in a common housing.
 17. The heating device according to claim 1, wherein the PTC heating element, the control device, valves for switching a supplementary circuit in and out, and a pump for producing a circulation of the liquid medium are arranged in a common housing.
 18. The heating device according to claim 1, wherein the motor vehicle is electrically propelled. 